This invention relates to cameras, shutters, and particularly to an exposure time control circuit to compensate for exposure errors ascribable to the overlap of curtains in a slit shutter.
Photographic cameras having slit shutters, such as focal plane shutters, control exposure times after a shutter release is actuated by initiating a timed interval when the leading shutter curtain starts to run. When the timed interval has passed, the trailing curtain of the shutter is released and begins to run. However, when a focal plane shutter is cocked, and prior to actuation of the release, the leading and trailing shutter curtains must overlap partially. The shutter is also susceptible to a response time lag in an electromagnet which controls the release of the trailing curtain. The difference between the positions from which the leading and trailing curtains start to run, and the response lag in the magnet make coincidence between the computed exposure time and the actual exposure time of the photographic material difficult if not impossible to achieve. As the requirements for exposure control accuracy become more rigorous, a control system which merely synchronizes the operation of the shutter and the timer circuit, so that a timing interval is initiated at the start of the movement of the leading shutter curtain and termination of the time interval causes movement of the trailing shutter curtain, has been unable to satisfy these needs.
For this reason, the conventional control system for the camera having the focal plane shutter such as those disclosed, for example, in U.S. Pat. No. 3,683,767 or Japanese laid-open patent application No. Sho 50-45628 has overcome the above-described drawback by providing addition of a certain adjusting time interval to the exposure time. In more detail, with the commonly available focal plane shutter when set in the initial position as shown in FIGS. 1(a) and 1(b), the rear border of the leading curtain LC of the shutter lies at a point FS1 which is different with reference to an image aperture IMA from a point rs1 at which the front border of the trailing curtain TC. In such arrangement, it is at a time t.sub.1 (see FIG. 1(b)) that the leading curtain LC starts to run down and also that a timed interval is initiated as shown in FIG. 1(c), and then when the timed interval Te (see FIG. 1(c)) is terminated at a time t.sub.2, an electromagnet (not shown) for holding the trailing curtain TC is actuated, causing the trailing curtain TC to run down. But it is actually at a time t.sub.3 as delayed by a response time Tmg of the electromagnet that the trailing curtain TC starts to run down. Further since there exists a time gap due to the differentiated locations of the borders of the leading and trailing curtains, the net exposure of the film amounts to a value TA which is not coincident with the timed interval Te, thus failing to satisfy the requirement for accurate exposure control.
That is, as is evident from (b) of FIG. 1, the response time lag Tmg depending upon the electromagnet, the time gap Tg depending upon the partial overlap of the two curtains, the timed interval Te and the actual exposure time TA have a relation as expressed by the formula: TA+Tg=Te+Tmg. In other words, the film is exposed to the imaging light for the time interval TA which is elongated from the timed interval Te by the timer circuit by the Tmg and simultaneously shortened therefrom by the Tg. And since these time intervals Tg and Tmg are unequal to each other, the ordinary cameras do not fulfill the requirement TA=Te. Therefore, it is necessary to otherwise synchronize the operation of the shutter curtains and the timing device. As the amount of overlap of the curtains in the initial position and the response characteristics of the magnet differ at random with different cameras, however, it is difficult to mechanically establish the prescribed precise synchronization. Because of this, the cameras proposed in the above-cited publications make use of a timer circuit in adding an adjusting time interval Tx to the timed interval Te so that at the termination of duration of Te+Tx, the trailing curtain control magnet is actuated to effect TA=Te.
That is, as shown on line (d) in FIG. 1, the termination of the timed interval Te is followed by the initiation of an adjusting time interval Tx, and then the trailing curtain control magnet is actuated with the result that the trailing curtain starts to run down at a time t.sub.4 as shown in the graph (b) of FIG. 1, and is allowed to take a move as as shown by a dashed line of the graph (b) of FIG. 1, thus obtaining the synchronization with Te=TA. In other words, since the conventional cameras are characterized by Tg&gt;Tmg, the TA tends to be shorter than the Te. This leads to the relation: TA+Tg=Te+Tmg+Tx. Then, Tx is made equal to Tg-Tmg by previously adjusting the time constant of the timer circuit. Thus, the timed interval Te and the actual exposure time TA are made to take the same value, and the above defined relation is realized.
It will be appreciated that the conventional phase error correction system for ensuring coincidence of the actual exposure time with the computed one by taking into account the shutter curtain overlap dependent time gap Tg and the response time lag Tmg of the magnet for individual each camera as these factors take different values with different camera is constructed by using the means for creating the adjusting time interval (Tx=Tg-Tmg) in a quite simple form of the timer circuit.
As the speed of running down movement of the shutter increases, however, when the time gap Tg related to the overlap of the shutter curtains becomes shorter than the response time lag Tmg of the magnet, it is impossible for such simple system to realize the establishment of Te=TA, resulting in exposure errors. In other words, as shown in a graph (a) of FIG. 2, when the shutter moves at so fast a speed that the time gap is shortened below the response time lag, the resultant adjusting time interval Tx takes a negative value. The principles of conventional method no longer hold to fulfill the requirement Te=TA.